1. Field of the Invention
The present invention relates to an image reading apparatus for a digital copying machine, a facsimile machine, a scanner, or the like, which has a simultaneous double-sided reading mechanism for simultaneously reading both sides of a conveyed original during one-time conveyance, and more particularly to an image reading apparatus having a reading magnification adjusting function.
2. Description of the Related Art
Conventionally, as an image reading apparatus for use in a copying machine or the like, there has been known one which performs so-called “moving original reading” in which originals are conveyed one by one onto an original platen glass by an automatic document feeder, and an original image on each of the originals is read by exposing the original to light by an exposure device fixedly disposed on the conveying path. Further, there has been known another image reading apparatus which is provided with two image readers for enhancement of productivity, and performs “simultaneous double-sided reading” for simultaneously reading both sides of an original during one-time conveyance of the original (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2004-187144).
In such an image reading apparatus which is capable of performing reading by the “moving original reading” method, a change in original conveying speed causes an error in original reading magnification. For this reason, it is required to causes an original to be always conveyed through an original reader of the image reading apparatus at a fixed speed.
For example, in a conventional general image reading apparatus, a moving original reading glass included in a reading optical system is disposed in a manner extending along a plane on which an original passes, and a conveying path is formed such that originals pass on the surface of the moving original reading glass.
Further, a plurality of rollers for conveying originals are arranged on the conveying path. More specifically, on an original entering side (conveying upstream side) of the conveying path upstream of the moving original reading glass, there are disposed rollers for stably conveying an original onto the moving original reading glass, and on an original discharging side (conveying downstream side) of the conveying path, there is disposed a discharge roller for discharging an original having been read, by friction between the original and the roller itself.
Now, a description will be given of the influence of a difference between a conveying speed of a lead roller 108 and a lead driven roller 109 of an image reading apparatus, described hereinafter with reference to FIG. 1, and a conveying speed of a lead delivery roller 111 and a lead delivery driven roller 112.
Assuming that the two speeds are set equal to each other such that there is no difference in conveying speed, if a difference in speed occurs e.g. due to a difference in finishing accuracy or wear, an error can occur in original reading magnification. Further, an original can be slackened and float from an original reading position, i.e. a position of a moving original reading glass 116 or 130, which adversely affects resolution or the like (i.e. which causes so-called “out-of-focus”).
To solve this problem, a technique described below has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H11-289420.
According to the technique, a conveying speed difference is provided between a conveying roller disposed upstream of an original reader in an original conveying direction and a conveying roller disposed downstream of the original reader. More specifically, the conveying speed of the downstream conveying roller is set to be higher than that of the upstream conveying roller. In a mechanism configured as above, the original is constantly pulled in the conveying direction at the original reader, which prevents the original from being slackened, thereby making it possible to achieve a stable and accurate reading operation.
According to the technique disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H11-289420, however, when an image reading apparatus is equipped with two image readers e.g. for the aforementioned “simultaneous double-sided reading”, the following trouble occurs:
Between the moving original reading glass 116 (see FIG. 1) for reading the front side (first side) of an original and the moving original reading glass 130 (see FIG. 1) for reading the reverse side (second side) of the original, there are differences in respect of timing in which the original is caught by the associated roller and timing in which the trailing edge of the original passes to leave the associated roller.
Therefore, when the conveying speed of the downstream conveying roller is set to be higher than that of the upstream conveying roller so as to achieve a stable reading operation, the sub-scanning magnification with which an original is scanned differs between the front side of the original and the rear side of the same.
Specifically, when the conveying speed of the downstream conveying roller is set to be higher than that of the upstream conveying roller as described above, a sub-scanning magnification with which a reverse-side image is scanned becomes smaller than a sub-scanning magnification with which a front-side image is scanned.